1. Field of the Invention
The present invention relates to thermoplastic polyester resin compositions having good low-temperature impact characteristics, improved thermal stability, and improved resistance to hot water and water vapor. More particularly the present invention relates to polyalkylene terephthalate resin compositions.
The resin compositions of the present invention are suitable for use in molding onto exterior parts, engine-room parts, electrical equipment parts, and electronic parts. Also, the compositions are useful in molding parts used in construction and other fields of activity and particularly, those parts which are likely to be exposed to a low temperature atmosphere.
2. Description of the Prior Art
Thermoplastic polyester resins, such as polyalkylene terephthalate resins, are widely used as engineering plastics in various industrial fields since they are recognized as having excellent mechanical and physical properties. A large number of such resin compositions have hitherto been proposed which are claimed to have improved properties to meet various characteristic requirements placed on such resins in the individual industrial fields.
While thermoplastic polyester resin compositions have some degree of impact characteristics and thermal stability there may still arise a problem with respect to impact strength if a part molded from such a composition has high notch sensitivity and a noticeably sharp rib or ribs.
In such case, a conventional approach is to add a thermoplastic elastomer to reduce the notch sensitivity. Among suitable elastomers which have conventionally been used are thermoplastic rubbers, such as graft copolymers wherein various vinyl compounds are grafted by using butadiene rubber as a rubber component and polyacrylate resins wherein a modifier composed of such a resin is a polystage polymer having a rubber-like first stage and a thermoplastic rigid final stage, polyolefins modified by block- or graft-copolymerization and/or ethylene-propylene rubber.
Although the addition of such a thermoplastic elastomer may considerably improve the impact resistance in a room temperature atmosphere, it will also result in considerable decrease in the impact strength in low-temperature atmospheres such as -40.degree. C. In these situations, large quantities of an elastomer having good low-temperature characteristics have been added. However, such an addition has been found to cause considerable deterioration in rigidity and other mechanical characteristics, with the result being that the composition no longer may be characterized as an engineering plastic.
Another difficulty with the prior-art compositions improved in such way concerns thermal stability. That is, during their short-time detention in the molten state in a molding machine, the compositions will suffer from deterioration in their physical properties because thermal decomposition will occur with the thermoplastic polyester resin itself and also with the thermoplastic elastomer. Furthermore, when a thermoplastic elastomer is added, recohesion takes place under the molten condition, thereby resulting in a considerable decrease in impact resistance.
As a counter-measure in such a case, normal practice has been to polymeric-plasticize the thermoplastic polyester resin to be used and/or to use a stabilizer. This practice has shown some measure of effect for retaining static strength characteristics such as tensile strength, but have almost no effect for retaining dynamic strength characteristics, such as impact resistance.
Therefore, the first aspect of the present invention relates to a solution to aforesaid problems from the standpoint of modifying the interface between the thermoplastic resin component and the thermoplastic elastomer component in a thermoplastic polyester composition, and also of inhibiting possible molecular weight loss due to thermal decomposition in the thermoplastic polyester resin.
Furthermore, although polyalkylene terephthalate resins are resistant to hot water to some degree, the application of these resins in car parts, which must be used in an atmosphere of hot water, has exhibited a problem in that the resins are hydrolyzed after a considerably short time thereby suffering deterioration in physical properties. In particular, tensile strength and elongation are adversely affected. Although the addition of phosphorus compounds, diene compounds, oxetane compounds or mixtures thereof to the polyalkylene terephthalate resins have been proposed in order to solve such a problem, no satisfactory solution has yet been reached.
Therefore, the second aspect of the present invention relates to solving the above problem by imparting hydrophobicity to the resin and simultaneously forming some network structures in the resin.